Atmospheric impact of isoprene-derived Criegee intermediates and isoprene hydroxy hydroperoxide on sulfate aerosol formation in the Asian region

This study revealed the importance of two isoprene-derived pathways in the formation of sulfate aerosol (SO₄²⁻(p)), which are related to the gas-phase reactions of Criegee intermediates (CIs) and liquid-phase reactions of isoprene hydroxy hydroperoxide (ISOPOOH), in the atmosphere in the Asian region, using the community multiscale air quality (CMAQ) modeling system. The results showed that the incorporation of both isoprene-derived CIs and ISOPOOH chemistry increased the SO₄²⁻(p) concentration in specific domains of Asia, particularly isoprene hotspots, with up to 1% of the total SO₄²⁻(p) concentration compared to that obtained by the basic chemical transport model. The impact of ISOPOOH to SO₄²⁻(p) formation exhibited a more localized pattern compared to that of CIs, attributed to the tendency of ISOPOOH to condense in low-NO regions. SO₄²⁻(p) formation from CIs affected the compositions of nitrate (NO₃⁻), ammonium (NH₄⁺), and secondary organic aerosols (SOA) in the particle phase, and in most of the analyzed sites, those compositions increased from 0.5% to 2.5%. In contrast, SO₄²⁻(p) formation from ISOPOOH exhibits a negative impact on SOA, resulting in a potential reduction of up to 5% due to the loss of SOA precursor by incorporating the ISOPOOH uptake reaction. The sensitivity analysis results showed that most of the SO₄²⁻(p) was generated by the gas-phase reaction of SO₂ with OH; however, the CIs and ISOPOOH also affected the SO₄²⁻(p) formation more than the liquid-phase SO₄²⁻(p) formations by H₂O₂, O₃, O₂, etc.